1. Field of the Invention
The present invention relates to an automatic temperature control circuit of a laser diode and an electric/optical signal converting unit using the same.
2. Description of the Related Art
An optical communication becomes indispensable according to that high speed and wide-band communication has been widely desired in recent years. In the optical communication system, the communication is performed by modulating an optical power of a laser diode with an electric signal, and sending the modulated light-signal via an optical transmission path in general.
Although the optical power of the laser diode is also used in other technical fields besides the optical communication, it is commonly expected that the optical power of the laser diode is stable, more particularly, for a temperature.
The laser diode employed in this explanation has a characteristic that the laser diode radiates when the value of current flowing to the diode exceeds the value of a threshold current (Ith). Further, the diode has another characteristic of outputting an optical signal modulated with a driving current obtained by piling an electrical pulse signal upon the threshold current (Ith).
The threshold current (Ith) employed in this explanation has a characteristic of varying depending on a temperature, because the laser diode is a semiconductor device. Further, it is general that there is a dispersion in a characteristic of each laser diode. Therefore, it becomes a problem that the optical power of each of laser diodes is different.
When showing these characteristic in the diagram, it becomes as FIG. 7. In FIG. 7, a driving current is shown in the axis of abscissas, and an optical power is shown in the axis of ordinates. One characteristic I shown in the diagram is that the threshold current is Ith, so that a bias current I.sub.B is supplied, and the signal current Ip is piled up to obtain the optical power P.
Further, if the temperature goes up, the characteristic I is changed to the characteristic II. Then, the threshold current becomes Ith', so that the bias current IB' is supplied, and the signal current further becomes Ip' to obtain the same optical power P.
Accordingly, an automatic optical power control circuit (APC) for detecting an optical power from a laser, and controlling the bias current and the signal current to make the optical power P constant has been provided in the conventional circuit.
Supposing that the relation between the threshold current Ith and the signal current Ip for obtaining a constant optical power has such a linearity characteristic shown in FIG. 8, in the automatic optical power control circuit (APC), control coefficients for the bias current and the signal current have been determined.
However, in the case where the temperature of the laser diode becomes higher than a predetermined value, according to the diversion of characteristic of the laser diode as described above, there is a possibility to have a characteristic shown in III of FIG. 7, for example, at the temperature that the threshold current becomes Ith'. In this case, the optical power becomes P', not P in the signal current Ip' which has same value as that of II.
Therefore, it is not sufficient that the relation between the threshold current Ith and the signal current Ip for obtaining a constant optical power is considered only on the basis of the linearity relation shown in FIG. 8. Thus, it was required to provide an automatic temperature control circuit (ATC) for controlling the temperature of the laser diode within a fixed range in the conventional circuit.
FIG. 9 shows a structural diagram of the conventional automatic temperature control circuit (ATC) prepared for above-explained objections. In the diagram, a temperature detecting section 1 comprises a thermistor 10 for showing an impedance corresponding to a temperature of a laser diode, not shown in the diagram, a variable resistor 11 connected to the thermistor 10 in series and a non-inverse amplifier 12.
Accordingly, an impedance of the thermistor 10 is changed in correspondence with the temperature of the laser diode, that is, the impedance of the thermistor 10 becomes smaller and the input voltage of the non-inverse amplifier 12 goes up, when the temperature goes up. An error amplifying section 2 comprises an operational amplifier 20 connecting an input resistor R1 and a feedback resistor Rf. A reference voltage V.sub.REF 21 is connected to one input terminal of the operational amplifier 20, and the output detected in the temperature detecting section 1 is inputted to other input terminal via the input resistor R1.
A differential voltage between the reference voltage V.sub.REF 21 and the output detected in the temperature detecting section 1 is outputted from the operational amplifier 20.
A current controlling section 7, comprising a pair of variable resistors 71 and 72, converts the voltage outputted from the operational amplifier 20 in the error amplifier 2 to the current, and the resistance values are variably controlled so as to equalize the size of the currents that flow through the resistors.
A peltier element driving section 3 comprises a class B push-pull power amplifying circuit composed of transistors 30 and 32 and transistors 31 and 33.
The class B push-pull power amplifying circuit is composed so as that the emitters of transistors 32 and 33 are connected in common, the emitter of the transistor 30 on the previous portion and the collector of the transistor 31 on the previous portion are connected to each of bases of the power transistor 32 connected to a collector resistance R7 and the power transistor 33 connected to a collector resistor R8.
Accordingly, if the temperature of the laser diode goes up, the output voltage of the non-inverse amplifier 12 becomes larger. Therefore, the output from the operational amplifier 20 in the error amplifying section 2 becomes larger toward the negative direction so that the transistor 31 becomes in the ON state and the power transistor 33 becomes in the ON state in correspondence with that the transistor 30 in the peltier element driving section 3 becomes in the OFF state and the power transistor 32 becomes in the OFF state.
Therefore, a cooling current in X direction flows to a peltier element 4, so that the temperature of the laser diode goes down. In opposite, if the temperature of the laser diode goes down, the transistor 32 becomes in the ON state, so that a heating current in Y direction flows to the peltier element 4. In this way, the current flowing to the peltier element 4 is controlled to keep the temperature of the laser diode constant.
The conventional temperature control described above will be further considered. As shown in FIG. 9, both ends of the push-pull output portion are composed of two sets of double transistors 30 and 32, and 31 and 33, respectively.
It is now considered that the case where the state of a set of output transistors 31 and 33 is changed from conductivity to non-conductivity, because of going up or going down of the temperature. It is required for making the output transistors 30 and 32 in the conductive state to generate the potential difference, which becomes more than the sum (V.sub.BE0 +V.sub.BE2) of the voltage V.sub.BE0 between the base and the emitter of the transistor 30 and the voltage V.sub.BE2 between the base and the emitter of the transistor 32, between the base of the transistor 30 and the emitter of the transistor 32.
Accordingly, as described in the temperature control characteristic a of the conventional temperature control circuit shown in FIG. 3, a dead band is generated when the state of conductivity is changed to the state of non-conductivity between one set of the transistors 31 and 33 and other set of transistors 30 and 32 of the output portions.
In FIG. 3, the axis of abscissas shows an ambient temperature of module for storing the laser diode, and the axis of ordinates shows the temperature of the laser diode. The temperature controlling characteristic a of the conventional circuit in FIG. 3 shows the situation of that the temperature of the laser diode is controlled so as to be 25.degree. C., when the ambient temperature becomes more than 25.degree. C.
Consequently, the temperature of the laser diode becomes less than 25.degree. C., when the ambient temperature of module becomes less than 25.degree. C., for the dead band existence.
On the other hand, does not depend on the consideration that how many amperes of the current should flow to the peltier element to control the temperature control circuit, that the ambient temperature 80.degree. C. is controlled to 40.degree. C., in the case where the setting temperature is 40.degree. C. and then the ambient temperature controlled to be 80.degree. C. That is, it is depends on the consideration that the temperature is stabilized by flowing a larger current for controlling so as to move to the setting temperature in an instant, cooling or heating, and bringing the value close to the adjacent to the setting temperature.
Therefore, it was apprehended to break down the circuit because the larger current flowed to the peltier element for controlling the ambient temperature to the setting temperature, in the case where there was a gap between the ambient temperature and the setting temperature when the power was supplied. In the conventional circuit shown in FIG. 9, the output of the operational amplifier 20 was saturated, and the variable resistors 71 and 72 in the current control section 7 were controlled variably to limit such a larger current.
Consequently, amplification rates of the transistors 30 and 31 on the latter portions, the V.sub.BE, the output of the operational amplifier 20 are dispersed, individually. That brings another problem that test steps are increased, because it is required to control those values, individually. Further, there is also a problem that a limited value of the current flowing to the peltier element is fluctuated because the voltage V.sub.BE between the base and the emitter of the transistors is fluctuated according to the temperature.
Meanwhile, a circuit module is placed on a test tool for performing the tests in the conventional circuit shown in FIG. 9. Therefore, it is apprehended that plus-minus power sources are individually supplied to the circuit module by a testing operator, and therefore, there is a possibility of the supply of only one power source.
In the conventional circuit shown in FIG. 9, for example, in the case where only -5.2 V is supplied when it is required to control toward the heating direction, the output from the error amplifying section 2 becomes a potential of approximately -4 V. And the bases of the transistors 30 and 31 on the previous portions in the peltier driving section 3 become low, the transistor 31 becomes in the conductivity state, so that the cooling current toward the X direction flows. Therefore, that brings other problem that the current flows in the direction reverse to the direction to be controlled.